05.14.09 – Most external flash units like the Canon 580 EX and the Nikon SB-900 have the ability to move the flash head forward and backward effectively zooming the flash. In normal situations having a flash zoom setting that matches the focal length on your lens is ideal. (When the settings match, the flash spreads the light over as much of the frame as possible.)
In some instances you might want to set the flash zoom to a longer focal length than your lens. This will darken the corners of the photo and enhance anything in the center of the frame.The Canon 580 EX II has a zoom range of 24mm – 105mm. The Nikon SB-900 has range of 12mm – 200mm.
For the most powerful output, speedlights should be set to the maximum zoom when using lighting modifiers like softboxes and umbrellas. For a largest light source possible, make sure the zoom setting on the speedlight is wide enough to cover the entire surface of the modifier.